Ataru Ichinose', George Daniels, Chau-Yun Yang and David C. Larbalestier. Applied Superconductivity Center, University of Wisconsin, Madison, Wisconsin ...
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCT'IVITY, VOL. 9, NO. 2, JUNE 1999
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Preparation and Characterization of Y z 0 3Buffer Layers and YBCO Films on Textured Ni Tape Ataru Ichinose', George Daniels, Chau-Yun Yang and David C. Larbalestier Applied SuperconductivityCenter, University of Wisconsin, Madison, Wisconsin 53706, USA. Akihiro Kikuchi Energy Conversion Material Research Group, National Research Institute for Metals, Tsukuba, Ibaraki 305-0047, Japan Kyoji Tachikawa Faculty of Engineering, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan Shirabe Akita Komae Research Laboratory, Central Research Institute of Electric Power Industry, Komae, Tokyo 201-85 11, Japan Abslract- The direct deposition o f Y203 buffer layers on cube-textured nickel tape was successfully performed by electron beam deposition using Y metal which oxidized during deposition. The Y2O3 layer exhibited excellent out-of-plane alignment of FWHM of 2.3 4 O and good in-plane alignment with -11 O FWHM. Surface morphology, crystal orientation and grain size proved to be quite sensitive to the deposition pressure. The surface roughness and the grain size increased with increasing deposition pressure, and the crystal orientation changed from ( l l l ) Y 2 0 3 to (1Oo)Y203. Subsequently, YBCO superconducting films were deposited on (1Oo)Y203 buffer layers by co-evaporation deposition and pulsed-laser deposition (PLD). Though a good in-plane alignment, as measured by X-ray $-scan ,was achieved in the YBCO films, their superconducting characteristics were not so good. The T, onset was about 84 K for the (001)YBCO by PLD. The crystal alignment and the microstructure o f YBCO superconducting films deposited by the two deposition techniques on cube-textured Ni tapes with Y203 buffer layers are compared.
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facilitated the production of the substrate for YBCO films by recrystallization of cold-rolled pure Ni tape by high temperature vacuum annealing.[5] Biaxially-aligned oxide buffer layers, for example, YSZ/Ce02, can be deposited on the RABiTS I\Ti tape by several kinds of deposition, such as PLDI, sputtering YBCO films deposited on both and e-beam deposition. types of metal tapes have Jc exceeding 1O6 A/cmZat 77 K in zero field. Their field dependence is reported to be very similar to films made on SrTi03 single crystals. We successfilly deposited biaxially-aligned Y ~ 0 3buffer layers on cube-textured Ni substrates by electron bean deposition. This technique easily produces the: Y z O ~buffer layers, needing only control of the deposition pressure. Subsequently, we also deposited YBCO films by COevaporation deposition and PLD. In this paper, we describe the crystallinity of the YzO3 buffer layers and the YBCO films deposited by these two deposition techniques. The superconducting properties of the Y B C O films were briejly investigated. 11. EXPERIMENT
I. INTRODUCTION
The fabrication of YBCO superconducting films on metal tapes is focused on the development of long and flexible conductors because of their potential for high critical current density of -IO6 A/cmZat 77 K. A most important issue is to decrease the density of high angle crystal boundaries. A crystal boundary with misorientation angle greater than -10 O behaves like a weak-link and the J, across such a grain boundary decreases significantly[l], [2]. In order to control the inplane alignment of polycrystalline YBCO films, several techniques have been developed. Biaxially-aligned yttriastabilized zirconia (YSZ) buf€ier layers have been grown on polycrystalline Ni-based alloy tapes by ion-beam-assisted deposition (IBAD) [3], [4]. A process called RABiTS has Manuscript received Sept. 14, 1998. This work benefited from EPRI, ORNL and NSF-MRSEC supported facilities. 'Permanent address: Central Research Institute of Electric Power Industry
The Ni metal substrates were prepared by rolling of ]Vi (99.7%) at room temperature followed by recrystallization annealing at 1O 6 Torr at temperatures between 600 and 900 "C for several hours. The detailed preparation of the substrates has been reported elsewhere[6]. The Ni substrates were shovvn to have a cube-texture with (001} Orientation. Tlie full-width at half maximum (FWHM) of the out-of-plane and in-plane grain distributions were typically -6" and -12", respectively. These results are shown in Fig. 1. Y203 buffer layers were deposited by electron beam deposition using an Y metal evaporation source. Tlie deposition temperature, which was measured on the heater side of the substrate mounting block, was 940 "C. Deposition of Y203 was conducted in both air (-lo5 Torr) and NZ ( 5 ~ 1 0 ~ Torr) atmosphere. In the deposition of Y ~ 0 buffer 3 layers at 5x1O4 Torr Nz atmosphere, frst we pumped down to 1O-' Tcirr air, before introducing Nz gas into the chamber tal control fhe pressure. We have already reported the YZ03 crystal
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alignment dependence of the deposition pressure[6]. It is easy to achieve the (1oo)Y~o~oriented film at a higher deposition pressure; however, the Ni is then easily oxidized. Consequently, Nz gas was used to deposit at high pressure and low oxygen partial pressure. YBCO films were deposited by both co-evaporation and PLD. In the case of co-evaporation,the temperature was 850 "C measured at the heater side of holder, similar to the case of a Y203 deposition. During deposition, pure oxygen gas was introduced near the substrate. After deposition, the films were annealed in 100 Torr of oxygen at T=500 "C. In the case of PLD, the deposition temperature was measured at the surface of the substrate using a pyrometer. The films were deposited at a temperature between 700 and 780 "C at 210
mTorr of O2atmosphere and then similarly annealed at -500 "C for 30 minutes.
The crystal orientation distributions for both YBCO and YzO3 were evaluated by X-ray diffraction, 8-28, o -scan and 4 scan. The surface morphology was observed by scanning electron microscopy (SEM). We measured the J, at zero field on bridges which were 200 pm wide and 0.5 mm long patterned by photolithography. 111. RESULTSAND DISCUSSION
Fig. 1 shows the X-ray rocking curves and 4 scans showing the out-of-plane and in-plane alignment for YzO~ buffer layers deposited at -1 O-' Torr air and 5x 1O4 Torr Nz, and the Ni
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0
I
(c) Ni(ll1)
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180
4 (dw)
270
360
-10
-5
0
5
10
e -8 peak (deg)
Fig. 1. XRD+scans (left) and rocking curves (right) showing the out-of-plane and in-plane FWHM for a Y203 buffer layer deposited at (a) 5x104 Torr N2 Torr air. The Ni substrate scans appear in (c). and (b)
Fig. 2. SEM micrographsfor Y203 films which were deposited at 940°C at (a) 5x104 Torr Nz and (b) -10" Torr air.
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substrate. The out-of-plane (FWHM 2.4-4O) for the Y203 buffer layer is much smaller than the -6" FWHM for Ni, indicating an improvement in the crystal orientation of the buffer layers. By contrast, the in-plane FWHM for Y203 and Ni are similar at about 11-12', indicating good epitaxial growth ofY203on Ni. Both Y203 buffer layers indicate good biaxial alignment, The SEM images of these films deposited at 5x 10" Torr NZ and -lo5 Torr air are shown in Fig. 2(a) and (b), respectively. The grain size and the surface roughness of the buffer layer grown in N2are much greater than for Y203 grown at los5Torr. The difference is a result of the different deposition pressure used during growth. The grain size and surfice roughness of Y203 increase with increasing deposition pressure[6]. In YBCO films deposited by co-evaporation, regardless of temperature, the formation of (001)YBCO was observed on the Y203 buffer layer deposited at -lo-* Torr air, but (103)YBCO grew on the Y203buffer layer deposited under 5x10"' Torr Nz. Our (001)YBC'O films have in-plane alignment of 11' FWHM
according to the (103)YBCO pole fiw shown in Fig. 3(a)1. This indicates that the (001)YBCO film grows epitaxially 011 the Y203buffer layer. In YBCO deposited by PLD, (001)YBCO was observed to form on the Y203 buffer layer deposited at -10" Torr air. An in-plane alignment of 10' FWHM was obtained. The (103)YBCO pole figure is shown in Fig. 3(b). In the case of the Y203buffer layer deposited at 5x10" Torr Nz, Y BCO films deposited at 730 OC and 760 OC have predominantly an a-axis and a (103) orientation, respectively. We also observed the surface morphology of both (100) and (001) YBCO films by SEM. These results are shown in Fig. 4(a), (b), respectively. The surface morphology of both filnis is very smooth. The grain shape for the (100)YBCO film is rather rectangular with strong grain alignment. The shorter length of the rectangular grains is about 0.1 pm, slightly larger than the Y203 grain size shown in Fig. 2(a). The (103) oriented grains also have a similar size. On (001)YBCO film, growth islands with well-defined terraces were observed by high
RD
TD
Fig. 3. (I03)YBCO pole figures of films deposited by (a) co-evaporationand (b) PLD.
Fig. 4. SEM micrographs OfPLD YBCO films on Y~03buffer layers deposited at pressures of (a) 5X1O4 Torr Nz and (b) -10" Torr air.
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resolution SEM as shown in Fig. 4@). The size of the islands ranged from 300 to 600 nm in mean diameter. The grain-to-grain connectivity does appear to be good. We measured the temperature dependence of the electrical resistance of PLD (001)YBCO films between room temperature and 77 K. The T, onset was about 84 K, indicating that the deposition conditions of the YBCO films have not yet been optimized. The J, of this sample was
